In the manufacture of sheet materials, it is well known that various sheet properties can be detected “on-line,” that is, while a sheet making machine is operating. On-line measurement devices measure sheet properties such as thickness, basis weight, moisture content, chemical composition and the like. Typically, such on-line devices employ sensors that periodically traverse, or scan, the moving sheets in the cross direction (CD), which is perpendicular to the machine direction (MD) of sheet travel. Depending upon the particular sheetmaking machine, cross-directional distances can range from about 10 to 12 meters and longer.
U.S. Pat. No. 4,957,770 to Howarth describes an infrared sensor to determine the amount of a coating material on a substrate using measurements of radiation reflected from a substrate or the transmission of radiation through the substrate, at two or more separate wavelength regions of infrared radiation. The infrared sensor includes an infrared source that transmits a beam of radiation toward the coated substrate such as a moving sheet of paper. When the beam reaches the sheet, it first passes through the coating material and then into the base paper sheet. A portion of the infrared energy is transmitted through the sheet while some of the infrared energy is reflected back in the general direction of the infrared source. In the case where the sensor is configured so that both the infrared source and receiver are positioned on the same side of the moving sheet of paper, the receiver measures the intensity of the reflected portion of the beam.
As is apparent, these standard single-sided sensors for measuring coat weight and other characteristics on layered products are configured for near normal diffusion scattering geometry that requires the product being measured to scatter light significantly so that a signal can be detected and analyzed. These single sided sensors are not especially suited for measuring thickness and weight of a coating that is formed on a reflective substrate.